1. Field of the Invention
This invention relates to preventing photo-induced damage, or photodarkening, to a laser material caused by unintentionally incorporated impurities. More particularly, the invention relates to preventing photodarkening in ytterbium-doped (Yb.sup.3+) optical fiber lasers and/or waveguide structures operating at a high optical output power level and which contain thulium (Tm.sup.3+) as an impurity. Photodarkening as a result of thulium contamination is effectively eliminated by co-doping the fiber core with other rare earth ions, preferably terbium (Tb.sup.3+), europium (Eu.sup.3+) and/or neodymium (Nd.sup.3+).
2. Description of the Prior Art
There has recently been growing interest in developing continuously operating (CW) laser sources with high output power and excellent beam quality. Most state-of-the-art devices of this type employ optical crystals doped with rare earth ions, such as Nd.sup.3+ :YAG emitting at 1.06 .mu.m, which are optically pumped either by flashlamps or, more efficiently, by semiconductor diode lasers. However, it is difficult to control the beam quality of lasers of this type over an extended output power range due to thermal instabilities and lensing effects, which result in spatial and modal instabilities of the output beam. Also, since the absorption peak of rare earth ions in crystalline host materials is rather narrow, the wavelength of the pump lasers has to be well controlled.
In order to obviate some of these disadvantages, fiber lasers doped with rare-earth ions and providing high optical lasing output powers have been developed. The geometry, in particular the waveguide characteristics of the fiber laser, advantageously overcomes some of the disadvantages mentioned above. In its simplest form, a fiber laser has a core which is doped with a rare earth ion providing a lasing transition when optically pumped, wherein the diameter of the core is preferably selected to permit either a single spatial mode (single-mode) or a controlled number of spatial modes (multi-mode) to propagate therein. The core is surrounded by a first cladding having a lower index of refraction than the core, with a second cladding surrounding the first cladding and having a lower index of refraction than the first cladding. The pump light is preferentially coupled into the first cladding either at one or both of the respective ends of the fiber laser structure, but may also be coupled in any other manner known in the art.
A high conversion efficiency from pump power to lasing output power has recently been obtained by carefully designing the geometry of the first cladding. Fiber lasers of this type are disclosed, for example, in U.S. Pat. No. 4,815,079 by E. Snitzer et al. and U.S. Pat. No. 5,533,163 by M. H. Muendel, both assigned to the applicant and incorporated herein by reference. By using a Yb.sup.3+ -doped fused silica core with a diameter of about 5 .mu.m for supporting only a single spatial mode and a substantially rectangular first cladding with a cross section of approximately 150 .mu.m.times.250 .mu.m for receiving the pump radiation generated by laser diodes emitting at about 0.915 .mu.m, a lasing output power in excess of 10 Watts CW, and more recently in excess of 30 Watts, was attained at a wavelength of approximately 1.1 .mu.m. Optical sources of this type are useful, for example, for applications in printing, material processing, and for pumping other fiber lasers, such as Er-doped fiber amplifiers for telecommunication.
Due to the small diameter of the fiber core, the optical flux, i.e. the optical lasing power transmitted per unit area of the fiber core, is extremely high. Consequently, absorption effects caused, for example, by impurities disposed either on the end faces of the core or inside the core itself, can result in unwanted degradation of the device within a time frame which is substantially shorter than the lifetime required for the respective application of such lasers. It was observed experimentally by the applicants that with constant pump power, the lasing output decreased by as much as several percent during a 100 hour time period. It was furthermore observed that Yb.sup.3+ -doped fiber lasers which exhibited such a substantial decrease in optical output, also emitted blue fluorescence at a peak wavelength of approximately 470 nm. Because Yb.sup.3+ ions have only one lasing transition, namely from the .sup.5 F.sub.5/2 (excited level) to the .sup.5 F.sub.7/2 (ground level) level, Yb.sup.3+ alone cannot be responsible for the observed generation of blue emission.
In a recent paper entitled "Frequency upconversion in Tm- and Yb:Tm-doped silica fibres" by D. C. Hanna et al., Opt. Communications Vol. 70, pages 187-194 (1990), it has been reported that Tm.sup.3+ - and Yb.sup.3+ :Tm.sup.3+ -doped silica fibers pumped at a wavelength of 1.064 .mu.m upconvert the pump radiation such as to generate blue fluorescence. Chemical and spectroscopic analysis performed on the Yb.sup.3+ -doped fiber lasers of the applicants indeed confirmed the presence of Tm.sup.3+.
In a paper entitled "Highly nonlinear near-resonant photodarkening in a thulium-doped aluminosilicate glass fiber" by M. M. Broer et al. which was published in Optics Letters, Vol. 18, No. 10, pages 799-801 (1993), it was reported that Tm.sup.3+ -doped silicate glasses exhibit photochromism, or photodarkening. The authors further stated that the rate at which photodarkening occurs, increases with increasing "pump" power when the fiber was pumped at a wavelength of 1.064 .mu.m. The authors postulated that multi-photon processes can raise the energy of the Tm.sup.3+ states to approximately 40,000 cm.sup.-1, which could result in the creation of color centers in the glass host. Color centers are known to cause optical absorption in the glass host.
Since photodarkening appears to be caused by impurity ions in the glass host, photodarkening could be prevented by using extremely pure starting materials for the manufacture of the fibers. However, the similar chemical properties and atomic masses of rare earth compounds make their purification rather difficult and expensive. This is particularly the case for Tm.sup.3+ and Yb.sup.3+ which occupy adjacent positions in the periodic table of the elements. Consequently, achieving a Tm.sup.3+ concentration of less than 1 part in 10.sup.9 in an Yb.sup.3+ doped fiber core may be remote. An alternate approach for preventing upconversion of lasing radiation and the optical absorption associated therewith, would be to neutralize the effect of such impurities. This could be accomplished by controllably introducing other ions into the glass host, in particular other rare earth ions, which would effectively quench or shunt, i.e. render ineffective, the upconversion process.
Co-doping of rare earth doped fibers is well known in the art. For example, in an Er-doped fiber laser co-doped with Yb.sup.3+, pump radiation at a pump wavelength of 930 nm is absorbed by the Yb.sup.3+ ions and subsequently transferred to the Er.sup.3+ ions for effectively providing amplification and/or lasing through an Er.sup.3+ transition in the wavelength range between about 1.53 .mu.m and 1.6 .mu.m. In U.S. Pat. No. 5,067,134 and in U.S. Pat. No. 5,617,244, there are described Tm.sup.3+ -containing fibers co-doped with other rare-earth ions, in particular with Tb.sup.3+, for efficiently upconverting pump radiation. Tb.sup.3+ is added to Tm.sup.3+ -containing fibers for facilitating de-excitation of electrons from the .sup.3 F.sub.4 level (labeled .sup.3 H.sub.4 in FIGS. 2a and 2b of the '134 patent) of Tm.sup.3+ to the .sup.3 H.sub.6 ground level, since population of the .sup.3 F.sub.4 level of Tm.sup.3+ would prevent the radiative transition from the .sup.1 D.sub.2 level to the .sup.3 F.sub.4 level, thereby blocking the emission of the desired 450 nm emission. The addition of Tb.sup.3+ is thus intended to make upconversion to blue more efficient.
As mentioned before, Broer et al. observed photodarkening in thulium-doped silicate fibers exposed to 1.064 .mu.m radiation. According to FIG. 4 in the Broer paper, the upconversion process cannot proceed if population of the .sup.3 F.sub.4 level in FIG. 4 (corresponding to .sup.3 H.sub.4 in the notation used by Broer) were effectively decreased by providing a de-excitation path from the .sup.3 F.sub.4 level to the ground level .sup.3 H.sub.6 of Tm.sup.3+.
It is, however, apparent to those skilled in the art that de-excitation of other energetically higher levels can also quench the upconversion process. De-excitation can therefore advantageously occur at any of the numerous energy states necessary for the upconversion process.